1. Field of the Invention
The present invention relates to a middle-ear prosthesis.
The natural mechanism for the transmission of sonic vibrations from the tympanum to the inner ear functions via the ossicular chain of the middle ear, consisting of three articulated ossicles--the malleus, the incus and the stapes, which amplify the vibrations of the tympanum for transmission of sound to the inner ear. The fenestra of the vestibule, in which the stapes (the smallest of the ossicles) lies, provides communication between the middle ear and the inner ear. The assembly comprising the tympanum and the chain of ossicles is commonly called the "tympano-ossicular complex".
Numerous pathologies of the middle ear (chronic otitis media in different forms, injuries, sequelae of previous operations) may result in dysfunction of the tympano-ossicular complex. This dysfunction entails a hearing impairment in the form of conduction deafness, that is to say impairment of the auditory function characterized by failure to achieve correct transmission of the sound message propagated from the external ear to the inner ear.
The middle ear functions essentially as an acoustic impedance transformer. If it cannot perform this function, the cochlear input signal is diminished and so-called conduction deafness results.
Such conduction deafness is principally associated with damage to the tympanum (perforation of the tympanum or with injury to the ossicular chain (ankylosis or ossicular lysis).
If the tympano-ossicular complex were entirely absent, the sound waves transmitted through the atmosphere would, without transition, encounter the base of stapes, in the fenestra of the vestibule, and would be reflected.
As a result, only a very small part (about one thousandth) of the sonic energy would be transmitted to the liquid of the inner ear.
This would mean that the patient would suffer a diminution of sensation of sound intensity perceived in the labyrinth of the inner ear. This diminution can be expressed as a loss in decibels.
It should be noted that the reconstruction of the various osseous parts of the middle or external ear is possible by means of ossicular alloplasty (ossicle taken from a donor) or by means of autoplasty (removal of one of the patient's ossicles, remodelling and restoration of tympano-ossicular continuity), or, more recently, by fitting a prosthesis made of a so-called biocompatible material. In recent years use has in particular been made of porous polyethylene (such as the porous polyethylene marketed under the trade mark Proplast.RTM.) or of biocompatible ceramics, particularly bioactive biocompatible ceramics (such as that marketed under the trade mark Ceravital.RTM.).
2. Description of the prior art
The reconstruction of the sound transmission mechanism in the middle ear by the implantation of biocompatible prostheses in well known.
It is in fact known to remedy different types of conduction deafness by replacing different parts of the ear, depending on the defect from which the patient suffers. The osseous frame of the external ear, the annulus surrounding the tympanum, the tympanic membrane itself, and each of the three ossicles of the ossicular chain can thus be replaced or repaired.
Depending on circumstances and on the type of surgical operation carried out, different categories of prostheses can be distinguished, for the production of which various materials may be used.
In Grote's U.S. Pat. No. 4,169,292 a prosthesis is described which is intended to replace the entire structure of the ear from the osseous canal of the external ear to the fenestra of the vestibule, which determines the frontier between the middle ear and the inner ear.
That part of Grote's prosthesis which is intended to replace the ossicular chain is fastened to an artificial ring, which in turn is joined to a tube replacing the osseous canal.
This type of prosthesis cannot be used if only the ossicular chain is defective or missing.
It is also known to reconstruct only the tympano-ossicular complex, or only the ossicular chain of the middle ear, or only a part of that chain.
A reconstruction of this kind can be made in the event of total excision. In this case the prosthesis is placed in a position different from that occupied by the natural ossicular chain in the cavity of the middle ear.
This is the case, for example, with Mercandino's U.S. Pat. No. 4,130,905, which describes a prosthesis consisting of a single piece replacing the entire ossicular chain, the position of the neotympanum being different from the normal position of the tympanum.
The use of a prosthesis of this kind entails a surgical operation completely changing the structure of the middle ear.
It is also known to reconstruct the ossicular chain of the middle ear when the position of the tympanum (naturel or neoformed) is retained.
There are two forms of prosthesis for the ossicular chain. When the stapes is intact, the prosthesis, which is then said to be partial, is fitted between the tympanum (or neotympanum) and the apex of the stapes. When only the base of the stapes is retained, the prosthesis is said to be total and is fitted directly between the tympanum (or neotympanum) and the base.
Total prostheses in use at the present time are generally prostheses functioning in the form of columellae, that is to say the entire ossicular chain (malleus, incus, stapes) is replaced by a single linear prosthesis interposed directly between the tympanum (or neotympanum) and the base of the stapes.
Examples of prostheses of this kind are described in U.S. Pat. Nos. 3,909,852 and 4,052,754 (Homsy), in 4,287,616 (Heimke et al.), and in 4,281,419 (Treace).
In the physical aspect, columellar type prostheses transmit sound directly from the tympanum to the base of the stapes and the tympanum is in direct contact with the prostheses.
In the normal ossicular chain the malleus is longer than the incus and is articulated to the latter by means of the incudomalleal joint. This has the consequence that this part of the chain serves as a lever arm permitting amplification of sound. This function is not served by columellar prostheses.
Furthermore, a second joint, known as the incudostapedial joint, exists between the malleus and the stapes in the normal chain.
The function of the joints is very important, because at one and the same time they permit smooth, progressive sound transmission and provide amplification through the lever arm system. This is not so in the case of columellar prostheses.